I'm really at the end of my tether with this.
I've had dogs all my life, mainly labs and collies who've house trained very quickly but also rescue dogs who've not been properly house trained as pups.
We now have an 8 month old boxer bitch.
She will not house train. The problem is she won't give any signals to be let outside. Everything has been done by the book, done again and redone again.
Lots of praise, treats, tethering to us, crate training, let out every 30 mins etc.
She continues to poo and we in her crate, or if she's not in the crate in the house.
I've contacted the breeder who says her mother was the same 
One of the other owners from her litter says she's having a lot of problems with her bitch too.
The vet says she has no health problems.
We have DC and caught out 18 month old walking around with dog poo in her hand the other week...
Just don't know what to do.